Interferometry has become a well known means for the accurate measurement of distance. Both electronic and optical means for increasing the resolution and accuracy of laser interferometers have contributed to advancements in the field until an accuracy of less than one part in ten million has been attained. However, as the resolution and accuracy of laser interferometers has been improved, consideration of certain error sources which are inherent to such devices has increased in importance. Temperature change errors, dead path errors, and errors due to the variable index of refraction of the air through which the laser beam must pass once represented only a small fraction of the available resolution. However, given the increased resolution of modern laser interferometers, which is in the range of a few nanometers, these error factors represent significant inaccuracies.
Laser interferometers commonly use an electronic apparatus for detecting and analyzing fringes created by phase differences between a measurement laser beam and a reference laser beam, and thus for detecting and analyzing indications of movements resulting in changes in length of a path traveled by the measurement laser beam. Temperature change errors result from the fact that the dimensions of optical components through which the laser beams must pass are altered slightly by variations in temperature. In prior art arrangements of interferometer components the reference beam path travels through a different length of glass within the interferometer as compared to that length of glass traveled by the measurement beam. Since the speed of propagation of the laser beams within the glass is different from that within the surrounding air, the reference and measurement beams are affected differently by the expansion and contraction of the glass components, thus causing some error.
Dead path errors result from differences between the path of the reference beam as compared to that of the measurement beam in that portion of those beams which preceeds the actual measurement portion of the beams, including those portions of the paths lying outside the glass optical components.
The index of refraction of air is dependent upon temperature and other physical variables and also upon the instant composition of the air, particularly the CO.sub.2 content. To the extent that the measurement beam is affected differentially by a change in the index of refraction of the air through which it travels, such changes in the air refractive index produces a false indication of a change of the length of the measurement beam path.
Given the increasing demand for increased resolution and increased accuracy of laser interferometers, clearly a means for eliminating or minimizing temperature change error, dead path error and/or variable air refractive index error would be desirable. Various means have been tried in the field to accomplish these goals. For example, it has been recognized that errors can be minimized by providing, as nearly as is possible, identical paths for the reference laser beam and the measurement laser beam. However, no satisfactory means has been found for accomplishing this goal. Further, it is now a well known practice to compensate mathematically for the variable index of refraction of the air. However, this practice is limited by the fact that no practical means has been found for quickly and accurately measuring the air refractive index in the environment using equipment which can also ultimately be used for taking the desired measurement.
It would be most desirable to provide equal length optical paths within the glass portions of the interferometer and further to minimize any differences between the paths of the two beams through the air. However, previous attempts to produce such a means have been less than successful. Also, it would be even more beneficial if the index of refraction of the air could be accurately measured in real time in order to compensate therefor.
All of the prior art interferometers within the inventor's knowledge have either incorporated optics in which the reference laser beam passed through an unequal length of glass as compared to the measurement laser beam and/or considerable differences has existed between the length of the paths of the two beams outside the glass portion of the optics.
No prior art interferometer to the inventor's knowledge has successfully provided a means for essentially eliminating temperature change error while reducing dead path error. Nor has any prior art interferometer to the inventor's knowledge successfully provided a means for using a measurement interferometer to also directly measure the prevalent index of refraction of the ambient air. All successful laser interferometer optics to date have suffered from excessive temperature change error and/or excessive dead path error and/or excessive error caused by the variable index of refraction of the ambient air.